1. Field of the Invention:
The present invention relates to small-diameter lined tubes which are resistant to interaction with chemicals conveyed therethrough and a method for making the same. More particularly, this invention pertains to composite tubing stock lined with a corrosion-resistant material and a method for successfully producing the same.
2. Discussion of the Relevant Art:
Small-diameter tubing, tubing stock having a diameter less than about 1 inch and more particularly between about 3/16 and about 3/4 inch, is used for a variety of applications. Among these are brake linings, fuel lines, and various conduits in a variety of non-automotive uses. It is necessary that these products be manufactured from durable materials which can withstand prolonged use as well as bumps, jostling and the like. In automotive applications, metals such as low-carbon steel are the materials of choice for reasons of economy and formability, for example. In other applications, other metals or various polymeric materials have been employed.
It is also necessary that the material of choice be essentially non-reactive with the materials conveyed within the tubing. Various fluids conveyed through the tubing exhibit degrees of reactivity with interior tubing surfaces. For instance, the components of various alternate fuels for example methanol contained in fuels such as M85 and ethanol in E85 can react with metals such as low carbon steel to cause corrosion. The resulting corrosion can weaken the tubing; eventually resulting in rupture, leakage and premature failure. Additionally, even materials which are not directly corrosive in a metallic environment can render the metal susceptible to exterior galvanic corrosion. Thus, the useful life of conventional low carbon steel is shortened even when conveying only mildly reactive materials.
Reactivity between the tubing and the materials conveyed within it can also compromise the purity of the conveyed fluid materials. Because of this, inexpensive metal tubing cannot be used in a variety of applications where possible fluid contamination with metal complexes, liquids or the like are not acceptable. In such instances, it is necessary to subject the tubing to post-formation processes such as annealing or to resort to more expensive tubing made of non-reactive metals or to tubing made from polymeric materials.
Polymeric tubing has been suggested as a substitute in various instances to eliminate the problems such as those previously discussed. Unfortunately, polymeric tubing presents a different set of problems. Conventional polymeric tubing is generally composed of materials such as vinyl, as polypropylene, polyvinylchloride and the like. These polymers exhibit poor formability characteristics. Because the polymeric materials exhibit elastic memory, the tubing constructed from such materials is difficult to permanently contour.
Prolonged use of sections of polymeric tubing can lead to the development of static charge. In this phenomenon, static charge builds up along the plastic line and, ultimately, results in numerous, unpredictable pinhole ruptures in the tubing. As can be appreciated, the use of polymeric lines is not desirable in high pressure applications or in situations in which the tubing will convey flammable liquids which could ignite upon escape during rupture. Furthermore, it is difficult to achieve burst strength characteristics in monolithic structures even if problems of static discharge can be obviated.
Even if the problems of formability and static discharge can be overcome, polymeric tubing, exhibit extreme weakness to heat. The polymeric materials employed in conventional plastic tubing sag, weaken or melt at undesirably low temperatures; rendering them impractical for many applications such as use in conjunction with automobile engines.
The materials employed in polymeric tubing also exhibit degradation over time due to interaction with other external environmental factors such as exposure to ultraviolet radiation which results in cross linkage, UV degradation and the like. This reduces tubing flexibility rendering the tubing brittle and easily breakable.
Finally, polymeric materials which make up the tubing can interact with certain organic components causing softening, localized deformation of the tubing, or permeation of the organic material through the polymeric material. Thus, a variety of organic fluids cannot be conveyed through polymeric tubing stock.
The use of composite or lined tubing stock composed of an outer metal layer and an interior non-reactive polymeric layer has been proposed. However, to date, only metal tubes having relatively wide inner diameters have been produced by spraying a liquid polymeric material onto the interior of the metal tube by means of an appropriate spray nozzle or other suitable dispensing device inserted into the interior of the metal tube to deliver molten polymeric material directly onto the interior wall. This method generally limits the production of such tubing to tubing stock having an inner diameter of sufficient width to permit insertion of the polymer dispensing device. Because the polymer applicator can only be inserted a relatively short distance into the metal tube, the overall length of lined tubing produced by this method is limited. Given these constraints, it can be appreciated that it has been impossible to employ the method to small-diameter tubing. Furthermore, it has been difficult to assure that the applied polymer plastic is uniformly dispersed over the interior surface of the tubing. Non-uniformities of the polymer deposited on the interior surface of the tubing can result in unwanted narrowing of inner diameter of the tubing causing constriction or blockage or insufficient polymer coating to prevent interaction with the reactive or corrosive fluids conveyed therethrough.
Thus, it is desirable to provide a tubing which exhibits enhanced resistivity to interaction with fluids conveyed therethrough. It is also desirable that this tubing be formable and machineable in subsequent post production operations. It is also desirable that this material contain a uniform non-reactive lining along its interior surface. It is desirable that the tubing be of sufficiently narrow diameter to permit its use in automotive fuel lines and the like. Finally, it is desirable provide an inexpensive effective method for producing such a material which eliminates the need for post-treatment processing steps such as annealing.